1. Field of the Invention
The present invention relates to a data communication apparatus and a data communication method that communicate by storing non-voice data in a voice packet, in a voice communication system that dialogues by exchanging voice packets via an IP network.
2. Description of Related Art
In recent years, so-called VoIP (Voice over Internet Protocol) communication system has rapidly become available. The VoIP dialogues by exchanging voice packets via an IP network. In addition, an IP telephone adaptor apparatus has been introduced, the apparatus enabling the continued use of an ordinary PSTN telephone in the VoIP communication system. The IP telephone adaptor apparatus enables a PSTN data communication apparatus (such as facsimile apparatus) to utilize the VoIP communication system.
The VoIP communication system includes a jitter buffer in a receiver, in order to buff the fluctuation of intervals between packet arrivals, the fluctuation being caused by delays within the IP network. Further, the VoIP communication system does not retransmit data to ensure the real-time voice reproduction. Therefore, when there is an overflow within the jitter buffer, the data is destroyed. When there is an underflow, the data is interpolated based on the data immediately before (see Related Art 1).
[Related Art 1]
Japanese Patent Laid Open Publication H11-88533
The above-described data destruction and interpolation during the overflow and underflow periods is effective for ensuring a quality telephone communication. However, such data destruction and interpolation should be avoided for facsimile data communication, since they may lower the communication quality by interfering an accurate data reproduction.
Since the fluctuation of intervals between packet arrivals, due to the delays within the IP network, is non-cumulative, the fluctuation can be absorbed by the capacity of the buffer to a certain degree. However, when the changes in the data amount stored in the buffer are cumulative, the jitter buffer cannot absorb the fluctuation by itself.
In particular, a slight difference in speed between the clock generators of the sender and receiver sides can cause a deviation of mutual process timings at both sides. For example, when the process timing of the sender side is faster than the one of the receiver side, the jitter buffer at the receiver side will gradually accumulate data, causing an overflow at the end. Conversely, when the process timing of the sender side is slower than the one of the receiver side, the jitter buffer at the receiver side will gradually lose data, causing an underflow at the end. These overflow and underflow problems, caused by the deviation of the mutual process timings, cannot be solved by the limited capacity of the buffer. Furthermore, it is not practical to largely increase the capacity of the buffer, since the manufacturing cost will also need to be increased.